The agent syslog write handler was unable to handle JSON log lines
correctly, meaning all syslog entries when using JSON log format
showed as NOTICE level.
This change adds a new handler to the Nomad agent which can parse
JSON log lines and correctly understand the expected log level
entry.
The change also removes the use of a filter from the default log
format handler. This is not needed as the logs are fed into the
syslog handler via hclog, which is responsible for level
filtering.
